DE 38 18 556 A1 discloses a magnetic bearing arrangement of this kind within a vacuum pump. It has as the motion element a rotor on which are mounted rotor blades that alternate with stator blades. Provided for lateral guidance of the rotor are two passive magnetic bearings, arranged at an axial spacing, that each comprise two concentric circular permanent magnets of which one is arranged on the rotor and the other on the stator, i.e. immovably on the unit, and which are magnetized in mutually repulsive fashion.
Arranged at the lower end of the rotor is a controllable magnetic bearing having an electromagnet that acts on an attraction disk attached to the rotor. The electromagnet comprises, in usual fashion, a magnetizable core constituting a stator element, and a coil comprising an electrical conductor. Excitation of the electromagnet is controlled by a stabilization device which has a position sensor to sense the axial position of the rotor. By means of the electromagnet, the rotor is held in a working position in which the permanent magnets of the passive magnetic bearings are displaced with respect to one another in such a way that they generate an axial force opposed to the attractive force of the electromagnet. The working position is provided in such a way that the electromagnet must always be excited in order to hold the working position. By varying the excitation, the stabilization device ensures that the rotor is always moved back into the working position in the event of an axial displacement
It is a disadvantage of this magnetic bearing arrangement that a control current is constantly required in order to hold the rotor in the working position by way of the electromagnets. In addition, stabilization is difficult because the magnetic force profile of the electromagnet is not linear.
German Unexamined Application DE-OS 29 19 236 discloses a magnetic bearing arrangement, for example for flow measurements, that has as the motion element a horizontally extending rotor. In order to hold the rotor in floating fashion, here again two passive magnetic bearings, each having a stator-side and a rotor-side permanent magnet that are magnetized in mutually repulsive fashion, are provided, the field components of the two passive magnetic bearings being directly oppositely.
Arranged in the gap between the two passive magnetic bearings is a magnet coil with which the magnetic fields of the passive magnetic bearings can be superimposed. Excitation of the magnet coil is controlled by a regulation device which includes position sensors that sense the axial displacement of the rotor in non-contact fashion and regulate the excitation current as a function of the axial displacement. The rotor is held in a specific working position depending on the direction and magnitude of the electrical currents in the magnet coil.
A disadvantage of this bearing is the large distance between the magnet coil and the permanent magnets attached to the motion element. The return forces achievable for stabilizing the working position of the motion element are correspondingly low. In addition, the magnet coil occupies the space in which, in the case of a rotor bearing arrangement, the rotary field stator for rotational drive would advantageously be placed.
It is the object of the invention to embody a magnetic bearing arrangement of the kind cited above in such a way that it is of the simplest possible configuration and consumes little power.
This object is achieved, according to the present invention, by way of the following features:                l) the controllable magnetic bearing has a permanent magnet;        m) the permanent magnet is arranged on the motion element opposite the stator element;        n) the magnetic force between permanent magnet and stator element is dimensioned such that with a control current of zero, the motion element is held in the working position; and that only upon deviation from that working position is a control current generated that influences the magnetization of the stator element in the direction of re-establishing the working position.        